1. Field of the Invention
The present invention relates to pulse doppler radar systems for measuring a distance to a target airplane by using a frequency modulation technique.
2. Description of the Prior Art
Radar systems for measuring a range to a target airplane by means of a transmitted wave which is frequency-modulated by a pulse frequency at a high repetition rate are well known. Such a radar system comprises: an exciter-receiver for alternately generating a linearly-changing-frequency modulated transmission signal so as to pulse-modulate the transmission signal at a high repetition rate to transmit the pulse-modulated wave from an antenna and for receiving an echo signal reflected by a target to amplify the received echo signal for eliminating clutter noises and to convert the signal to a digital signal, said antenna amplifying the transmission signal to transmit it into the atmosphere and receiving the echo signal reflected from the target; and a signal processing unit for receiving the digital signal from the exciter-receiver to effect various steps of processing of the digital signal to obtain a value indicating the range to the target.
The transmission signals from the exciter-receiver include repetition of a set of signals consisting of a constant-frequency portion and a linearly-changing-frequency modulated portion and are pulse-modulated at a high repetition rate for transmission from the antenna. The antenna amplifies the pulse-modulated signals to provide high power signals which are in turn delivered to a multiplicity of antenna elements (not shown). The antenna elements add predetermined phase shifts to the signals and transmit them into the atmosphere so as to form antenna beams that are steered electronically for the purpose of searching for targets. An echo signal from a target is received by the antenna with clutter noises.
FIG. 1 shows the positional relationship between a target and a radar system mounted on a flying body. In this figure, the reference numeral 1 designates a radar system; 2 the main beam of the antenna; 3 the side lobes; 4 a target; and 5 the ground. In the case where radar system 1 is moved at the speed V.sub.I and searches target 4 in a down-look condition, radar system 1 receives unnecessary clutter noises because main beam 2 and side lobes 3 illuminate the ground. The signal from target 4 received by main beam 2 includes a plurality of doppler frequencies due to modulation caused by rotation of the jet engine compressor blades of target 4.
FIG. 2 is used to explain how planar waves input to the jet engine compressor blades are subject to the doppler shift in the negative direction. In this figure, reference numeral 6 designates the blades, and target 4 is assumed to be flying in the direction shown by arrows A opposite to the direction of propagation of the planar waves shown by arrow B. Since blades 6 rotate in the direction shown by arrow C such that the blades 6 would move away in the direction opposite to direction A from the illuminating plane of the planar waves, the doppler shift tends to become low. In addition, such factors as complicated shapes and differences in the angles of inclination and speed of rotation of the blades 6 result in the generation of subharmonic components of the modulated wave.
More specifically, the wave now impinging at point P.sub.1 on the blades 6 will impinge at point P.sub.2 on the blades 6 after a short period during which the blades 6 rotate through a small angle. Accordingly, blades 6 act as if to move away from the radar system 1 even though the target 4 is approaching it. As a result, a plurality of doppler frequencies lower than the doppler frequency based on the relative speed V.sub.R between target 4 and radar system 1 are produced.
The doppler frequency produced by a target airplane flying at the speed V.sub.T is expressed by the following equation: ##EQU1## wherein f.sub.o indicates a transmission frequency; C.sub.o the light velocity; and .theta. the angle between the speed vector V.sub.I and the antenna beam.
Assuming that the amount of frequency shift due to jet engine modulation is indicated by f.sub.JEM, the plurality of doppler frequencies are expressed as follows: ##EQU2##
FIG. 3 shows a frequency spectrum of the clutter noises and the echo signal received by the radar system in the condition shown in FIG. 1. In that figure, reference numeral 7 designates main beam clutters; 8 side lobe clutters; 9 the target doppler frequency signal; and 10 doppler signals due to jet engine modulation. The received echo signal includes the real doppler frequency signal 9 determined by the speed relative to the target and given by equation (1), the jet engine modulation signals 10 having a frequency lower than the real doppler frequency and given by equations (2)-(5), the main beam clutter 7 and the side lobe clutters 8.
Receiving the signal from the antenna, the exciter-receiver amplifies the signal in a low noise condition, converts the amplified signal to an intermediate frequency signal by way of a local oscillation signal, eliminates the noises from the intermediate frequency signal, obtains doppler frequency signals in the video band by phase detection and converts the doppler frequency signals to digital signals. These digital signals are input to the signal processing unit which then eliminates the noises again and converts the noise-eliminated digital signals to narrow-band doppler frequency signals.
FIG. 4 shows changes with time in the frequency of the transmitted and received signals which are frequency-modulated for the purpose of measuring the range. In the figure, the reference numeral 11 designates a curve indicating changes in frequency of the transmitted signal; 12 a curve indicating changes in frequency of the target doppler frequency signal; and 13 curves indicating changes in frequency of the doppler frequency signals due to jet engine modulation. As shown in this figure, a plurality of doppler frequency signals are detected in FM phase C where the frequencies are constant and FM phases B and A where the linearly-changing-frequency modulation is effected. In FM phase C, four doppler frequency signals respectively having the frequencies f.sub.1, f.sub.2, f.sub.3 and f.sub.4 are received, and in FM phase B four doppler frequency signals are detected having the frequencies g.sub.1, g.sub.2, g.sub.3 and g.sub.4, respectively, which are frequency-shifted in accordance with the range to the target. Also in FM phase A four doppler frequency signals respectively having the frequencies h.sub.1 h.sub.2, h.sub.3 and h.sub.4 which are frequency-shifted in accordance with the range to the target are received.
The frequency of the target doppler signal is higher than the frequency of the transmitted signal by f.sub.1 and is delayed by the time given in the following expression: EQU .tau.=2R/C.sub.o ( 6)
wherein R indicates the range to the target.
Similarly, the frequencies of the jet engine modulation signals are higher than the frequency of the transmitted signal by f.sub.2, f.sub.3 and f.sub.4, respectively, and are delayed by .tau..
If one doppler frequency signal is received in each of FM phases C and B, the target range is calculated as follows using frequency f.sub.1 detected in FM phase C and frequency g.sub.1 detected in FM phase B: ##EQU3## wherein F indicates the degree of frequency modulation in FM phase B. Then ##EQU4##
If, on the other hand, two doppler frequency signals are received in each of FM phases C and B, the problem is that four range values are obtained because four frequency differences are obtained. In order to overcome this problem the conventional radar system employs an additional FM phase A in which the degree of frequency modulation is different from that of FM phases B and C, and three-phase ranging is effected to obtain the range to the target.
In a case where, as shown in FIG. 4, three doppler frequency signals or more are received in each FM phase, it is impossible to obtain the real range value even if three FM phases are used.
Thus the calculation by equation (8) is done using FM phases C and B in the case of one doppler frequency signal being received in each FM phase, and using FM phases C, B and A in the case of two doppler frequency signals being received in each of FM phases C and B.
In a case where three doppler frequency signals or more are received, there are a multiplicity of combinations of frequencies, as shown in FIG. 5. If N signals are received, there are N.times.N combinations of frequencies. Since the frequencies in FM phase B are always lower than those in FM phase C, the values of R.sub.21, R.sub.31, R.sub.32, R.sub.41, R.sub.42 and R.sub.43 are all negative and are thus excluded from the calculation. In reality, there are (N+1).times.N.div.2 range calculations. This forces the conventional radar system to decide that the calculation necessary for obtaining the real range value is impossible in this case. In other words, although there is only one target to be detected, the range calculation is determined to be impossible in a case where a plurality of doppler frequency signals are received due to jet engine modulation. This leads to a significant deterioration in the performance of radar systems.